Sen. John McCain (R‑Ariz.) says his proposed campaign finance restrictions are a sure winner in the Senate. His only concern is that Democrats might abandon ship. McCain has reason to worry: Democrats might well end up killing his pet cause.

This prediction seems absurd. After all, congressional Democrats have been firmly united behind campaign finance restrictions for reasons of ideology and self-interest. Why would they turn against “reform” just as it seems likely to pass?

Because the McCain-Feingold legislation will change the political universe in two ways, neither of which helps Democrats.

First, it will ban the unregulated “soft” money that goes to the political parties. Despite liberal howling about soft money, the fact is Democrats now pull in almost as much of it as Republicans. In 2000, the Democrats raised $199 million (49 percent of all soft money) while the Republicans garnered $211 million (51 percent of the total). Democrats will be loath to give up this potent political weapon at the moment they’ve achieved parity with the GOP.

Second, it will prevent labor unions from paying for radio and TV ads favoring Democratic candidates or issues. Democrats will think twice before disarming their friends.

Past votes by congressional Democrats don’t tell us much about what they might do this year. Votes for McCain’s earlier proposals were always “free votes” for Democrats — they could vote “yes” knowing that the bill would not become law. But voting for McCain-Feingold now means it might actually pass. Once Democrats focus on the real consequences of the legislation, they ‘ll experience a foxhole conversion.

To illustrate why, consider the case of Chuck Robb, the former Democratic Senator from Virginia. During his two terms in the Senate, Robb supported most gun-control measures. The National Rifle Association targeted him for defeat in 2000 in his race against George Allen. Acting independently of Allen’s campaign, the NRA spent $572,000 against Robb as part of its “Vote Freedom First” campaign. Robb lost with 48 percent of the vote.

Isn’t this an argument for why Democrats will support campaign finance reform? No, because spending by groups like the NRA would not be restricted by the McCain proposal. If the courts uphold the ban on soft money, the Democrats will face the worst of all possible words — no more soft money coming into their coffers even as independent groups like the NRA are free to raise and spend as much as they like.

What’s more, banning soft money won’t eliminate it from the political system. It will simply be redirected from the political parties to independent groups like the NRA, swelling their budgets. Surely some Senate Democrats are now wondering, “How will I fend off attacks without the help of soft money? How do I avoid becoming Chuck Robb?”

So a number of Senate Democrats will conclude that McCain-Feingold offers few advantages and potentially large headaches. They’ll want to vote against the bill. The only problem will be how to backpedal on campaign finance reform after years of ardent support. Enter the White Knight: George W. Bush.

Bush will insist that McCain-Feingold include a strong “paycheck protection” provision to prevent unions bosses from forcing their members to support political positions they may not agree with. Since the unions hate “paycheck protection,” Senate Democrats can help their allies by voting against campaign finance reform. Publicly, they’ll lament that they were forced to take this course of action. But in private, they’ll be relieved. The “poison pill” of paycheck protection will taste sweet.

Of course, the real winners will be the American political system and the Constitution. Motivated by self preservation, Senate Democrats will end up preserving the First Amendment right of the American people to spend money propagating their political beliefs. Maybe bipartisanship isn’t so bad after all.