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Within hours of Supreme Court Justice Antonin Scalia's death on Saturday, Senate Majority Leader Mitch McConnell said that the Senate should not confirm anyone whom President Barack Obama nominates to fill the vacant seat, but wait until a new president is elected.

McConnell's comment put in bold relief the huge stakes, not just of the presidential election, but of who controls the Senate. If Democrats take back the Senate as well as the White House, a Democratic president could replace not only Scalia but also fill two and possibly three other Supreme Court seats likely be vacated in the next few years.

Election watchers believe that there is a reasonable chance that the Democrats can gain four seats and take back the Senate. (Republicans currently have a 54-46 Senate majority. If Democrats win the White House and gain four Senate seats, they control the Senate because the vice president breaks a 50-50 tie.) A more liberal court could overturn Citizens United, restore voting rights and workers' rights, protect women's right to abortion, allow the president to address climate change, keep affirmative action in jobs and education, and deliver many other important rulings.

In fact,

Scalia's death will have immediate consequences on pending court decisions, because rulings that would have been decided by 5-4 margins will now end up as 4-4 ties, which means that the lower courts' rulings will stand.

Labor union leaders and allies were bracing for the court to cast an anti-union vote in Friedrichs v. California Teachers Association, which would have ended public sector unions' ability to require employees to pay fees to cover the costs of collective bargaining, even if the workers benefited from these activities. Such a ruling would have been devastating to the labor movement, but now the ruling of the Ninth Circuit Court of Appeals, which favored the unions, will stand.

Scalia's death also means that two cases that involved legislative redistricting-which would made it easier to redraw districts to favor white, rural, Republican voters-will now revert to the lower court rulings, which opposed the Tea Party plaintiffs' effort to draw lines based on voters rather than the entire population. The Supreme Court was going to hear Zubik v. Burwell, in which several religious groups sought to block an Obama administration regulation requiring faith-based organizations to sign a form asking for an exemption from providing contraception coverage to employees as part of their insurance plans, as mandated by the Affordable Care Act. The lower courts ruled against the nuns and other religious groups, so a 4-4 Supreme Court ruling would give Obama a victory for women's health.

The Supreme Court balance could shift even before the November election if the Republican Senate approves Obama's nominee to replace Scalia, but given McConnell's remarks, that is unlikely. And even if the Republican Senate were willing to confirm Obama's nominee, it would only be for a judicial moderate and not a full-throated liberal like Ruth Bader Ginsburg.

If the Democrats win the Senate and a Democratic president gets to replace Scalia and appoint three other justices, they will cement a liberal majority for at least two or three decades. If either Clinton or Sanders wins the White House, Justices Ginsburg (who will be 83 next year) and Stephen Breyer (78) might retire to allow the president to pick their younger successors. Anthony Kennedy, a conservative who sometimes votes with the court liberals, will be 80 in 2017. If he retires and a Democrat selects his replacement, the court could find itself with a 6-3 liberal majority, with only Chief Justice John Roberts (currently 61 years old) and Justices Clarence Thomas (67) and Samuel Alito (65) remaining to carry the conservative torch. (Two other liberals-61-year-old Sonia Sotomayor and 55-year-old Elena Kagan, both Obama appointees-could remain on the court for another two decades)

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Even with Roberts remaining as chief justice, a court with a 6-3 liberal majority could have more influence in moving the country in a progressive direction than at any time since Chief Justice Earl Warren led the court between 1953 and 1969.

Democratic leaders thus hope that progressives will put as much time, energy, and money into helping Democrats win Senate seats in the key "battleground" states this November as they do helping a Democrat win the White House.

Political prognostication is hardly a science, but a number of analysts have looked closely at the 34 Senate seats up this year and identified those where the races will likely be very competitive. Drawing on forecasts by Sabato's Crystal Ball, Roll Call, National Journal, Charles Cook Report, and the Rothenberg/Gonzalez Political Report, there appear to be between nine key battleground states where a small number of votes could mean the difference between a Democratic or Republican victory in November. In two other states, Republican incumbents could be in trouble but are currently favored to win re-election. And in Indiana, the Republican incumbent is retiring, which could result in a competitive race.

Among the nine most competitive states, seven Senate seats are currently held by Republicans. In one of them, incumbent Marco Rubio is running for president and not seeking to keep his Senate seat. Two of the battleground states are now held by Democrats. In Colorado, Democratic incumbent Michael Bennett is seeking re-election. In Nevada, Democrat Harry Reid is retiring, so the seat is wide open.

Here's the rundown of the key battleground states:

New Hampshire: First-termer Kelly Ayotte is probably the most vulnerable Republican in the Senate. She's facing a strong opponent in popular Democratic Governor Maggie Hassan, who announced her Senate bid in October. New Hampshire voters have supported Democrats in five of the past six presidential races. This gives Hassan an edge. Possible Democratic pickup.

Wisconsin: Incumbent Ron Johnson is another vulnerable Republican seeking re-election. The billionaire invested about $9 million of his own money to beat Senator Russ Feingold by a small margin in 2010, a midterm election. Feingold is now seeking to regain his former seat and has the advantage of this being a presidential year, where Democratic turnout is likely to be higher than six years ago. Obama carried Wisconsin with 53 percent of the vote in 2008 and 56 percent four years later. Possible Democratic pickup.

Illinois: Republican Mark Kirk rode the GOP wave to victory in 2010, but this year he's facing a tight race for re-election in a state where voters typically support a Democrat for president and where the other Senate seat is held by Dick Durbin, a popular Democrat. Representative Tammy Duckworth is likely to be the Democratic candidate for Senate. An Iraq War veteran, Duckworth served as a U.S. Army helicopter pilot and suffered severe combat wounds, losing both of her legs and damaging her right arm. She was elected to Congress in 2012 and re-elected two years later. Right now she is leading Kirk in most of the statewide polls, and Kirk is considered the underdog. Possible Democratic pickup.

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Colorado: Democrats believe it is crucial to hold onto this Senate seat, currently held by Michael Bennet, who is running for re-election. He was appointed to that seat in 2009 by Governor Bill Ritter when Ken Salazar became secretary of the Interior. Bennet won the seat on his own in 2010, narrowly defeating Republican Ken Buck. There is no clear frontrunner among Republicans seeking the party's nomination, which gives Bennet an advantage. Obama carried Colorado in both 2008 and 2012, but it is still considered a swing state in the current presidential race. Bennet will need a strong Democratic turnout to stay in office. Tossup.

Ohio: Republican incumbent Rob Portman is running for re-election. His likely Democratic opponent, former Governor Ted Strickland, is currently leading Portman in the polls. Strickland won a landslide victory for governor in 2006 but lost a close race to John Kasich four years later. This will be an intense battleground state in both the presidential and Senate races. Tossup.

Pennsylvania: The Republican incumbent Patrick Toomey wants to stay in the Senate, but he is not a very popular politician in this state. The two leading Democrats are Katie McGinty (former chief of staff to Governor Tom Wolf) and former Representative Joe Sestak, whom Toomey narrowly defeated six years ago as part of the GOP wave. Pennsylvania will also see a highly competitive race for president, even though a GOP presidential candidate hasn't won Pennsylvania since 1988.

A strong Democratic turnout could doom Toomey's re-election bid and help the Democratss take back the Senate.

Possible Democratic pickup.

Nevada: Democrat Harry Reid, who has served in the Senate since 1987 and was its majority leader from 2007 to 2014, is not seeking re-election, so this is a wide open seat. Reid was lucky that in 2010 the Republicans nominated Tea Party extremist Sharron Angle as their Senate candidate. She was an ineffective campaigner and Reid beat her by a 55 percent to 45 percent margin, but many analysts believed he would be vulnerable to defeat this year if the Republicans put up a better opponent. After bowing out, Reid recruited former Nevada Attorney General Catherine Cortez Masto to run for the seat as the Democratic nominee. She will likely face Representative Joe Heck in the general election in what promises to be one of the most competitive Senate races in the country. Tossup; possible GOP pickup.

Florida: This is another state where the incumbent is not running for re-election. Marco Rubio is seeking the GOP nomination for president, leaving the seat vacant. Florida will be one of the most hotly-contested states for both president and Senate. Both parties hold their primaries on August 30, which will make this a prolonged battleground state. Until recently, it looked like Representative Alan Grayson, a charismatic progressive, had the edge to win the Democratic nomination, but he now faces a scandal over his business practices, having operated a hedge fund while serving in Congress. Several of his key campaign staffers have resigned. Some top Democrats, including Senate Minority Leader Harry Reid, have called on Grayson to quit the race. If he does, that would make Representative Patrick Murphy the favorite to win the Democratic nomination. He'll have a slight edge over any of the likely GOP candidates, who include Lieutenant Governor Carlos Lopez-Cantera, Representatives Ron DeSantis and David Jolly, and defense contractor Todd Wilcox, a former Special Forces commander and CIA veteran. Tossup; possible Democratic pickup.

Arizona: It is possible that Arizona voters are getting tired of Republican John McCain, who has served in the Senate since 1987 and was the GOP's losing presidential nominee in 2008. Political handicappers give McCain an edge but predict that he'll have the toughest re-election fight of his career and could be defeated if the Democrats nominate a strong candidate and invest the money needed to run a good campaign. He is likely to run against Representative Ann Kirkpatrick, the toughest Democratic challenger he has ever faced. A strong Democratic turnout, especially among women, could give Kirkpatrick a victory. Tossup; longshot Democratic pickup.

In two additional states, Republican incumbents-Missouri's Roy Blunt and North Carolina's Richard Burr-could face tough re-election bids, but the political prognosticators think these Senate seats will remain in GOP hands. In Indiana, Republican incumbent Dan Coats is stepping down, but it will be difficult for a Democrat to win that open seat unless they come up with a very strong candidate and voter turnout among low-income, minority, and young voters reaches record levels.

Bottom line: In a high turnout election, Democrats have a better-than-even chance for a net pickup of at least four seats. Filibusters are still allowed to block Supreme Court confirmations. However, with a newly elected Democratic president and Senate, it's not clear that Republicans would take that risk, especially since rules can be changed with a simple majority. We all knew how consequential this year's election will be. With Scalia's death, it just got even more consequential.