ANALYSIS — I have argued repeatedly that while the House is up for grabs — and indeed likely to flip to the Democrats in November — the Senate is not in play. I now believe that it is, so I must revise and extend my remarks.

Only about three weeks ago, I reiterated my view that Democrats didn’t have a path to a net gain of two Senate seats, which they need for a chamber majority. But a flurry of state and national polls conducted over the past few weeks suggest Democratic prospects have improved noticeably, giving the party a difficult but discernible route for control.

Democrats are at least even money to flip two GOP-held Senate seats in November — Arizona and Nevada. Both races are very competitive, but President Donald Trump’s problems, the midterm dynamic and the two states’ fundamentals — Trump lost Nevada and barely carried Arizona — surely give Democrats a narrow but clear advantage in both states.

Months ago, I acknowledged a long-shot Democratic opportunity in Tennessee, largely because of the reputation of their high-quality nominee, former Gov. Phil Bredesen. But I didn’t take Bredesen’s chances against the Republican nominee, Rep. Marsha Blackburn, seriously given the state’s partisan bent and Trump’s strong showing there two years ago.

I’m still skeptical about Bredesen’s prospects, but recent polls show that the race is for real. I can no longer simply dismiss his chances.

Unlike some, I still have trouble imagining Rep. Beto O’Rourke upsetting Texas Republican Sen. Ted Cruz. But Trump’s problems in some suburban areas of Texas in 2016 and his upcoming trip to the state to boost Cruz’s prospects certainly suggest that the Lone Star State should be on the radar. The challenger is still a distinct underdog, of course, but the race deserves some attention.

ICYMI: Democrats Could Gain Up to 8 Governor Seats in 2018

Democratic edge

Two vulnerable Democratic incumbents — Joe Manchin III of West Virginia and Joe Donnelly of Indiana — now appear to be narrowly ahead in their races.

West Virginia GOP nominee Patrick Morrisey is a relatively weak challenger, but GOP insiders warn that the state’s demographics and Trump’s strength there mean it’s too early to count the challenger out. Still, there is little doubt that Manchin currently has the edge in the contest.

Donnelly led GOP businessman Mike Braun by 6 points in a recent NBC News/Marist survey, and if the Democrat votes to confirm Brett Kavanaugh, as many expect, it could be enough to prove the senator’s “independence” to swing voters and even some Republicans.

Braun must raise the incumbent’s negatives and rally Republicans behind his effort to get some momentum.

Republicans believe Wisconsin Democrat Tammy Baldwin remains vulnerable and cite a recent Marquette poll that showed Baldwin’s lead within the margin of error. But other polls have found Baldwin in much better shape, and the state Supreme Court race earlier this year, as well as Republican Gov. Scott Walker’s weak poll numbers, confirm that the Badger State is a problem for the GOP. Any midterm breeze helping the Democrats should keep the Senate seat in their column.

In Montana, Democrat Jon Tester remains ahead, though Republicans note Trump’s strong following in the state. Tester’s re-election got off to a strong start, but GOP insiders hope that the president can rally his supporters behind challenger Matt Rosendale, the state auditor.

Good for GOP?

If those were the Republicans’ best prospects for takeover, Democrats would indeed have reason for great optimism. But three other Democrat-held seats are at the greatest risk, which still make the Democratic task of winning the Senate very difficult.

Veteran handicappers agree North Dakota Democrat Heidi Heitkamp is in serious trouble, though they don’t yet agree with Republicans who think the race is essentially over. Heitkamp is again running a good race, but her Republican challenger, Rep. Kevin Cramer, doesn’t have the baggage that her 2012 opponent did, and even her admirers worry that the state may be too Republican, too conservative, too white, too rural and too pro-Trump for her to win another term.

Meanwhile, Florida’s Bill Nelson and his Republican challenger, Gov. Rick Scott, are running even in two recent statewide surveys, while Missouri’s Claire McCaskill and her opponent, GOP state Attorney General Josh Hawley, are tied at 47 percent in a recent NBC News/Marist survey.

Trump carried Florida very narrowly but won Missouri by almost 19 points.

Nelson, 75, was first elected to the Florida House in 1972, and he has run for office repeatedly in the ensuing 45 years.

McCaskill, 65, was first elected to the Missouri Legislature in 1982. She rode a Democratic wave to the Senate in 2006 and was surprisingly re-elected six year later when the GOP nominated a weak challenger.

Neither Democrat has the statewide strength of Manchin or Ohio’s Sherrod Brown, but so far, they are holding their own.

It’s likely that the midterm dynamic is a major factor in the closeness of those races.

Scott is over-performing among Hispanic voters now, and his deep pockets may be enough to help him squeeze out a narrow win. But it is also possible that a strong African-American turnout (to support Democratic gubernatorial nominee Andrew Gillum) and Republican midterm problems among seniors and college-educated white suburbanites could benefit Nelson.

The GOP’s 5-point advantage in the generic ballot question in Missouri and Trump’s strength in the state are obvious problems for McCaskill, as are her already high personal negatives. But even with those liabilities, she is hanging in against Hawley.

The good news for the GOP is that it still has many opportunities to oust Democratic senators. The bad news is that a handful of those targets look worse off now than six or 12 months ago, while the Democrats have added a race or two worth watching.

Republicans remain upbeat about their chances, and they should be. They are still more likely than not to retain control of the Senate. Democrats need almost everything to go right to net two seats.

But during wave elections, tight Senate contests often all fall in the same partisan direction — and if Tennessee, Florida and Missouri do just that, there is a certainly a path for Democrats. It’s just a very narrow and rocky one.