The fun begins at 10 p.m. ET, streaming live online at WISN.com. If you’re looking for a reason to watch, here you go: Realistically, the only thing that might happen between now and Tuesday to shift the conventional wisdom about the outcome would be a horrible debate by Walker tonight or a surprisingly strong performance by Tom Barrett. The second-most important election in America this year may turn on what happens over the next two hours. If that’s not good enough, watch so that you’re well prepared for your blog reading next week. Monday, Tuesday, and Wednesday will be a tsunami of Wisconsin-related posts.

Robert Costa, who’s been tracking the race, identifies four things to watch tonight. The key factor:

Barrett is trying to make the election about Walker’s celebrity among conservatives across the country, tying the governor to the tea-party movement and “far right” interests. He has largely avoided promising to peel apart Walker’s fiscal reforms, which remain more popular than Democrats like to admit. This has been a boon to Walker, but in the final debate, look for Barrett to hit harder.

In the first debate, Walker was mostly disciplined as Barrett sniped, saying again and again that the recall was initially “about our reforms” but that Democrats have pointedly changed topics since “the reforms are working.”

But if Barrett comes armed with talking points about the budget, Walker will have to keep steady and engage him. Walker’s best bet, GOP operatives say, is to bring up how his reforms are helping public schools to make financial decisions free of the public-sector unions’ heavy hand. This argument makes the issue easy to understand.

Elsewhere, Philip Klein argues that the recall election on Tuesday plus the SCOTUS ruling on ObamaCare later next month make June 2012 the most pivotal moment for America in a long, long time:

Should Walker win, the ramifications will go far beyond Wisconsin. Walker’s ability to survive a well-funded recall effort in a bluish state that is likely to vote for President Obama would send a message to governors in cash-strapped states throughout the nation. They will be more likely to confront their unions, too.

Beyond that, a Walker win would prove to politicians at the state and federal level that voters will not only tolerate but reward leaders who take a stand on “third rail” issues and remain firm in the face of protests, threats and special-interest scare tactics.

Because Walker followed through in implementing his reforms, Wisconsin parents with school aged children had a full academic year to see that none of the dire warnings the unions had made about his policies ever came true. Instead, local governments saved money with the added flexibility the reforms gave them, and property taxes went down for the first time in many years.

A victory will be encouraging but I’m less sure than Klein is that other GOP governors will be suddenly emboldened by it. For one thing, John Kasich lost badly when he tried to reform Ohio’s collective bargaining law so Walker’s victory is no guarantee of universal success. Also, losing in Wisconsin could compel unions to redouble their efforts and prove their strength by going full steam at the next Republican governor who pushes reforms. Some GOP leaders, like Chris Christie, might relish that fight. Others might not, and the fact of Walker’s victory might not do much to encourage them. After all, how many governors want to spend a chunk of their term swamped with having to defend a recall bid, with all the fundraising and campagining that that requires? Even the lure of becoming a national conservative rock star a la Walker might not help much since the next governor who goes after public employee unions will be seen as following in his footsteps, not blazing a trail. The greater value in winning on Tuesday, I think, isn’t the effect it’ll have on Republican leaders but the effect it might have in moving the Overton window for some voters nationwide who otherwise never would have considered the possibility of remaking collective bargaining laws. They’ll learn a simple lesson: Wisconsin tried it, the world didn’t end, and in fact things turned out well enough that they retained the guy responsible as governor when they had a chance to dump him. As that process of reform “normalization” begins among the wider public, I think that in turn will start to embolden governors. But how long that process will take, I don’t know.

While we wait for the debate to start, enjoy this bit of speculation about how a Walker win might blow up in Obama’s face five months from now. Exit question: Say, didn’t noted Tom Barrett supporter Bill Clinton used to oppose recall elections? Quote: “I don’t want you to become a laughingstock, a carnival or the beginning of a circus in America where we just throw people out whenever they make a tough decision.”

Update: From last night’s “Hannity,” here’s the man of the moment pleading his case nationally.