The battle for control of the US Senate just got hotter.

Gov. Maggie Hassan (D) of New Hampshire announced Monday that she’s running for Senate, taking on freshman Sen. Kelly Ayotte (R). Governor Hassan was the Democrats’ top choice for the race. The nonpartisan Cook Political Report immediately shifted its ranking of the race to “tossup.”

With Hassan’s long-awaited decision, “Democrats scored perhaps their biggest Senate recruiting success of the cycle,” writes Jennifer Duffy, Senate analyst for Cook.

The Democrats need a net gain of five seats in the 2016 elections to retake control of the Senate – and only four if a Democrat wins the White House. The playing field favors the Democrats, who are defending 10 seats to the Republicans’ 24 seats. Cook’s ratings show only two vulnerable Democratic seats (only one of them a tossup) versus seven vulnerable Republican seats, including four tossups.

“Ayotte's vulnerability can be attributed largely from having to run in a swing state in a presidential year rather than anything she has or hasn't done,” writes Ms. Duffy.

Still, Senator Ayotte is in a tough spot, and both candidates are going to have to run the race of their political lives to succeed. Ayotte has an advantage as the incumbent, albeit a first-termer. Hassan’s advantage comes from the fact that she’s running during a presidential election year, when key Democratic constituencies – single women, young voters, and minorities – turn out in higher numbers than in off years.

The presidential race could play a crucial role in determining who wins the New Hampshire Senate seat. If the Democratic nominee is strong, she or he could inspire turnout that benefits Democrats further down the ballot. The reverse could also be true: A strong Republican nominee could help Ayotte.

In the Hassan-versus-Ayotte smackdown, polls matching them up have shown a tight race. Three polls this year by the Democratic-leaning Public Policy Polling (PPP) found them in a tossup, 44 percent for Ayotte, 43 percent for Hassan. But Hassan has a higher job approval rating than Ayotte: Hassan has 48 percent positive, 42 negative. Ayotte has 38 percent positive, 46 percent negative.

But the wider political environment doesn’t always hurt a vulnerable candidate. Last November, Sen. Jeanne Shaheen (D) of New Hampshire won reelection in a bad year for Democrats.

Still, Ayotte’s challenge from Hassan represents one more piece of bad news for the Republicans, forcing them to spend money in a race they hoped wouldn’t be so tough.

Among Democratic-held seats, Cook’s only tossup is the seat held by retiring Senate minority leader Harry Reid of Nevada. In that race, Rep. Joe Heck (R) faces former state Attorney General Catherine Cortez Masto (D).

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On the Republican side, aside from Ayotte’s seat, three others are tossups:

Sen. Mark Kirk of Illinois, who has health issues and is running an uphill battle in a deep blue state. Rep. Tammy Duckworth (D) is his top opponent.

In Florida, Republican Sen. Marco Rubio’s decision to give up his Senate seat to run for president has unleashed fierce nomination battles on both sides of the aisle for the race to replace him.

Wisconsin’s Senate race probably features the most vulnerable Republican this cycle. Freshman Sen. Ron Johnson (R) faces the man he defeated in 2010, former Sen. Russ Feingold (D).

For Democrats, retaking the Senate is seen as an insurance policy against a potential Republican presidential victory. The Republicans have a strong grip on the House of Representatives, and if they keep the Senate and win the White House, the Democrats’ only hope of checking the Republicans is through filibusters and other procedural actions.

Senate control is also crucial for high-level executive branch nominations, including the US Supreme Court. The next president could nominate as many as three justices.