Sen. Dan Coats (R-Ind.) announced Tuesday that he will not seek reelection in 2016. It was not an unexpected announcement, but it does set in motion a potentially competitive open-seat race in the battle for the Senate.

And the competitiveness of that race relies in large part on one number: $9.9 million.

That's the ungodly sum of money that retired Sen. Evan Bayh (D-Ind.) has just sitting in his campaign account. Bayh succeeded Coats when Coats retired in 1998, and Bayh returned the favor by retiring and making way for Coats in 2010. In both cases, there was a sense that the retirements weren't entirely voluntary, but rather were about the difficulty of the campaign ahead. (Bayh was a popular recent former governor in 1998; by 2010, he had to answer for his vote on things like Obamacare in an increasingly red state and a bad year.)

Bayh has certainly left his options open. While he has said he won't run for governor in 2016, he hasn't ruled out a return to politics. And that $9.9 million war chest is so big that most incumbent senators would be overjoyed to have it. Combine it with Bayh's moderate reputation, and it's little secret how happy the Democratic establishment would be to have him in the race.

The question is whether Bayh kept that money in the bank for just such an occasion. The former senator and governor, after all, is a relatively young 59. And while he has been talked up as a potential presidential candidate or VP pick in the past, that ship has largely sailed, it would seem. So the money largely has one place to go: Another run for Senate.

Here's what he told the Huffington Post's Sam Stein and Paul Blumenthal in October when they asked him about why he was sitting on money that could have been used to help Democrats defend the Senate:

On Thursday, Bayh told The Huffington Post that he had been generous with his remaining campaign funds, having transferred roughly $1.5 million over the last three years to other Democrats. Part of the reason he wasn't doing more, he said, was that he was advised "by former colleagues who retired and had given significant money to some of the committees" that those donations went unappreciated. But the other reason he wasn't doing more was less personal and more political.
"I'm in my 50s. Most of the other people [holding on to cash] are in their early 70s," said Bayh. "So I don't know what the future might hold. I don't think it makes a lot of sense closing doors." Asked if he was planning a future run for office, he added, "I think the chances of that are not high. But you just never know."

The early conventional wisdom is that Bayh probably doesn't run. And that might be true. Democrats will also note that they won a seat here in 2012, so it's not like getting the big-name recruit is a requirement.

But their 2012 victory was largely because of what happened on the GOP side -- specifically, state Treasurer and sayer-of-unhelpful-things Richard Mourdock beating incumbent Sen. Richard Lugar in a primary and losing to then-Rep. Joe Donnelly (another moderate Democrat) in the general election.

Barring another disastrous GOP nominee, Democrats will be hard-pressed to win again. Former congressman Baron Hill has been mentioned as a possible candidate, among others. But in a presidential year in a state that Mitt Romney won by 10 points, they'll need some help.

That $9.9 million would be pretty helpful.