Senate Democrats know they may lose their last congressional tool to block legislation from the GOP-led Congress in November 2018, because they must defend 23 Seats seats while the GOP need only defend eight seats.

That leaves many of the 23 Democratic Senators — and their leaders — with the difficult task of balancing party solidarity and personal political survival. Party leaders commonly give vulnerable senators leeway to buck their party going into an election year, but if the Democrats allow a third of their senators to freelance on critical votes during the next two years, they cease to function as a party at all.

For newly installed Senate Minority Leader Sen. Charles E. Schumer (D.-N.Y.), this means playing a most dangerous game with 10 of his colleagues up for reelection in states won by President Donald J. Trump — plus, Maine, where Trump won one electoral vote.

The choice for Schumer and the Democrats is whether to fiddle in the middle, blurring the distinctions between its incumbents and the Trump-led Republican Party, or make a last stand at Fort Obama, defending former president Barack Obama’s legacy — even at the cost of losing the ability to delay and or block legislation.

The magic number is eight. With eight more seats, Senate Republicans no longer need worry about Democrats.

Coming out of the 2016 election cycle, Republicans hold a 52-to-48 seat majority, with the Democrats holding 46 seats combined with two Independents who caucus with Democrats. Upon the Jan. 20 swearing in of Indiana Gov. Michael R. Pence as vice-president, the GOP will be able to sustain two defections, having Pence on stand-by to break a tie.

Inside the Senate, a minority faction has the power to block legislation because there is a 60-vote requirement to formally end debate and hold a vote on legislation and nominations to the Supreme Court. With 41 votes, a bloc can keep the debate going forever and thus block any bill from coming to the floor. That ability to block legislation, however, does not apply to bills dealing with the budget, which is why Republicans are using the budget track to repeal the 2010 Patient Protection and Affordable Care Act, Obamacare.

If the 2018 election sends eight more Republicans to the Senate, the Democrats lose their last vestige of the maximum power they commanded in 2009-2010, when they held 60 votes in the Senate from Sept. 24, 2009 to Feb. 4, 2010, plus control of the House and the White House.

Nine of the Democrats up in 2018 are freshmen, so they are coming into their first defense of their seat, typically the most difficult. Here they are with their 2012 percentage: Sen. Christopher Murphy (D.-Conn.), 55 percent; Sen. Angus King (I.-Maine), 53 percent; Sen. Mazie K. Hirona (D.-Hawaii), 63 percent; Sen. Joseph Donnelly (D.-Ind.), 50 percent; Sen. Timothy Kaine (D.-Va.), 53 percent; Sen. Martin T. Heinrich (D.-N.M.), 51 percent; Sen. Elizabeth Warren (D.-Mass.), 54 percent; Sen.Tammy Baldwin (D.-Wis.), 51 percent and Heidi K. Heitkamp (D.-N.D.), 50 percent.

Six of the nine won their seats in 2012 with 53 percent or less of the vote: Baldwin, 51 percent; Kaine, 53 percent; King, 53 percent; Heinrich, 51 percent; Heitkamp, 50 percent; and Donnelly, 50 percent.

These are the first four seats the Republicans need to pick up in 2018. All but Heinrich and Kaine are from states where Trump won an electoral vote. Trump lost New Mexico to his Democratic rival Hillary R. Clinton, 48 percent to his 40 percent, but freshman Heinrich is still vulnerable given there is a Republican governor and the state has been deep purple for many cycles now. Virginia is competitive and purple — the Republican Party in the commonwealth does not have the same Red Zone killer instinct of years gone by.

Of the nine freshmen, Baldwin, King, Heinrich, Heitkamp, and Donnelly are the first five seats the GOP needs to pick off towards eight.

Two veteran Democratic Senators kept their seats with 53 percent or less: Sen. Jon Tester (D.-Mont.), 49 percent, and Sen. Sherrod Brown (D.-Ohio), 51 percent. Brown already has an opponent, Ohio Treasurer Josh Mandel, the Republican and Marine veteran of Iraq who ran against Brown in 2012. Tester should have had to face Rep. Ryan Zinke (R.-Mont.), but Zinke, a retired Navy SEAL officer and combat veteran, was selected by Trump to lead the Department of the Interior. Zinke, as the only congressman in the state, ran statewide like the senators, so he already had experience campaigning beyond a small district. But that ship has sailed.

Of the two Democrats who kept there seats with 53 percent or less in 2012, only Brown is vulnerable. That brings the total to six towards the eight.

The next batch of targets will require some work.

The National Republican Senate Committee needs to recruit strong candidates and the president needs to get involved.

There are four more Democratic senators from states Trump won and each has had tough races in the past, so there is a very strong possibility of picking off: Florida’s Sen. C. William Nelson II, Missouri’s Sen. Claire McCaskill, Pennsylvania’s Sen. Robert P. Casey Jr., and West Virginia’s Sen. Joseph Manchin.

With these four seats, the GOP would then gain ten.

But, there are two sides to this ledger. The GOP has eight seats they must defend and not all of them are in good shape.

The weakest of the eight in the 2012 cycle were Sen. Dean Heller (R.-Nev.), 46 percent, and Sen. Jeff Flake (R.-Ariz.), 49.7 percent. Both men were hostile to Trump during the 2016 presidential campaign, with Flake actually having a shouting match with Trump when he visited with GOP senators. Heller might catch a break in 2018, as Trump won Arizona and lost Nevada.

There is every chance in the world that Flake and Heller return to private life in January 2019.

Thus, 10 minus two equals eight.

Eight plus 52 equals 60–and gives the Republicans the super-majority votes they need to dismiss all challenges from Senate Democrats.

If the Republicans do not seize the chances 2018 offers, which really came about because of their horrendous results in 2012, it could be six years before the chance presents itself again. In the 2020 Senate cycle, the Republicans defend 22 seats to the Democrat’s 11, a reflection of the GOP’s nine-seat pick up in 2014. Then in 2022, the GOP defends the 24-to-10 results it just won in November.

Democrats have held 60 or more seats in 12 different congresses, including 1937-to-1939, when they dominated the Republicans 75-to-17. Republicans have never held 60 seats in the Senate, but twice they held 59: 1909-to-1911 and 1921-to-1923.

An eight-senator pickup would be an historical mark, and it would mean in practical terms that Republicans cannot be stopped by anyone but themselves.