WHO WILL WIN THE SENATE?

The Republicans are hanging onto the majority by a thread. Could the Democrats take back the upper house this November?

WASHINGTON — The Democrats could let a golden opportunity slip through their fingers on Election Day.

This year’s battle for control of the Senate should favor the blue party. The Republicans hold just a five-seat edge, and most of the 34 seats up for election or re-election are in states that are either tossups in the presidential contest or lean to the left.

Yet with a little more than five weeks to go, Republicans are in control of two key Senate races — in swing states Florida and Ohio — and are holding their own in three more crucial contests in the battlegrounds of New Hampshire and Nevada and the blue state of Pennsylvania.

If the Republicans maintain their seats in Florida, Ohio, New Hampshire and Pennsylvania and flip the seat in Nevada left by retiring Senate Minority Leader Harry Reid, their odds of keeping their majority for the next two years rise substantially.

Bolstering their prospects is a $21 million infusion into these and other key Senate races to fund an ad blitz, according to The Washington Post. The investment is being made by the Senate Leadership fund, the super PAC overseen by Senate Majority Leader Mitch McConnell.

The GOP’s majority in the Senate stands at 54-46 (including two Independents that caucus blue), and the nonpartisan Rothenberg & Gonzales Political Report projects the Democrats could flip anywhere from three to six seats this year. That means the upper chamber is still in play, whereas the party’s majority in the House appears secure.

If Hillary Clinton defeats Donald Trump in the race for president, Democrats would only need to win four more seats because their party’s vice president would serve as the tie-breaker in a 50-50 Senate. Currently Clinton holds a modest lead over Trump following their first debate.

And, in the most competitive Senate races, Democrats could still enjoy a turnout advantage. In two contests — in the blue states of Illinois and Wisconsin — the Democratic candidates are now clear favorites. In the red state of Missouri and the purple state of North Carolina, Democratic nominees are running surprisingly strong against entrenched GOP incumbents.

And then of course, there’s the Trump factor. In August, it appeared Trump might sink the GOP majority in the Senate and possibly even the House.

Trump is the most unpopular nominee of a major party in the history of modern polling, and he remains vastly unpopular with nonwhite voters and white moderates. The Republican nominee remains a major challenge for the party’s Senate candidates to navigate.

But since Labor Day, Trump has run a more disciplined campaign and reduced, though not eliminated (see Alicia Machado), his penchant for offensive, controversial rhetoric.

Simultaneously, Clinton, who is only slightly more popular than Trump, has endured fresh attention for payments made to her family’s charitable foundation and her use of a private e-mail server.

These two factors have led to a more competitive Senate race, which has allowed the hard work of Republican Senate candidates to pay dividends. Whether that continues remains to be seen. This election season has been unpredictable and is likely to remain so right up until November 8.