Former senator Evan Bayh (D-Ind.) (AP)

In our list of top 10 most competitive Senate races in 2016, Republican-held Indiana was not on it. That will probably change if, as expected, the state's best-known Democrat, Evan Bayh, gets in the race, while his party's lesser-known nominee drops out. (And to be clear, as of Monday afternoon he hasn't officially entered the race.)

Indiana Democrats' surprise game of musical chairs is big news for not only the open Indiana Senate race (GOP Sen. Dan Coats is retiring), but for Democrats' chances to take back control of the entire Senate. In a map full of opportunities for Democrats, they just got another one.

To understand why, let's break down the news in Indiana. As CNN's Tom LoBianco first reported Monday morning, former congressman and current Democratic Senate nominee Baron Hill said he'd stand down to make room for Bayh.

"Democrats have a very real chance at winning this Senate seat, especially with a strong nominee who has the money, name identification and resources to win," Hill said in a statement.

As Hill spelled out, Bayh (pronounced "By") does indeed tick all three boxes for a candidate in a competitive race:

1. He's well known in the state: Bayh was Indiana's governor for almost a decade and then a popular U.S. senator for more than a decade. (His father was a U.S. senator, too.)

2. He's suffered no major controversies: Bayh won his two Senate elections with more than 60 percent of the vote. His 1998 election with 64 percent of the vote was the largest percentage of the vote ever recorded for a Democrat in a Senate race in Indiana.

3. He has lots of cash: This is by far the biggest plus in Bayh's column. When Bayh unexpectedly retired in 2010, he took with him a war chest of some $10 million. That means he has almost 10 times as much cash on hand as the Republicans' Senate candidate, Rep. Todd Young, who's got a little more than $1 million. A Senate Democratic operative said that kind of money will go a long way to insulate Bayh from whatever attacks Republicans throw at him (more on that in a minute).

Before Bayh was in the picture, most nonpartisan political watchers expected the seat to stay in Republican hands. Democratic nominee Hill's fundraising hasn't been great, and he had already lost a race to Young — a former Marine and congressional staffer before he ran for Congress — when Young kicked Hill out of his House seat in 2010.

That was then. Now, for the reasons we just mentioned, two nonpartisan political analysts just moved the Indiana Senate race from favoring Republicans to a tossup or even favoring Democrats.

And that means Republicans will have one more seat to worry about in their already stretched-thin efforts to hold on to their majority this fall. (Of the 34 U.S. Senate seats up for grabs in November, they're defending 24.)

Meanwhile, Democrats need to pick up four (or five, if Donald Trump wins the presidency) to take back the majority. And even before Indiana, they were set up with the best possible chance to do that for four big reasons:

1. Eight of the 10 races most likely to flip parties on our most recent ranking are Republican-held.

2. A recent Quinnipiac poll shows that Senate races in key swing states are in play (though the most recent poll, released in late June, gives the Republican candidates in Florida and Pennsylvania a clear edge).

3. Republicans recently lost one of their two pickup opportunities when Colorado Republicans nominated a candidate who Senate Republicans back in Washington think is toxic.

4. Donald Trump.

But Indiana is not going to be a shoo-in for Democrats. President Obama scraped by with a win in 2008 but lost by 10 percentage points in 2012.

Nor is Bayh going to be a shoo-in. While Bayh is a known quantity to many Indiana voters, he's also a known quantity to Republican researchers, who hope to hit him on stuff like this:

(MacGillis, FWIW, is a reporter for the nonpartisan ProPublica, but Senate Republicans' campaign arm sent out a statement saying as much Monday afternoon.)

When Bayh left the Senate six years ago, he took a big swipe at it. His race has the potential to be a mirror image of Marco Rubio's reelection campaign: The guy who reportedly doesn't like the Senate is running to get back into it. This from a New York Times op-ed Bayh wrote in 2010 will almost certainly be used against him:

"Today, members routinely campaign against each other, raise donations against each other and force votes on trivial amendments written solely to provide fodder for the next negative attack ad. It’s difficult to work with members actively plotting your demise."

Still, for a candidate who suddenly puts a race in play just by deciding to enter it, Democrats will take their chances. And his $10 million war chest.