In the presidential campaign so far, the Democrats have more money, more momentum and more optimism. The Republican field by comparison is sluggish. Unless a new candidate emerges or one of those already in the race can break out of the doldrums and energize the electorate, GOP hopes of holding on to the White House will diminish.

If a Democrat takes over 1600 Pennsylvania Avenue in January 2009 and the party maintains control of Congress, it could usher in a big change for workforce policy—well, any policy—because Republicans would lose the safety net of a presidential veto on legislation it opposes.

So that means that the action will focus on the Senate even more so than it does today. Democrats maintain a 51-49 margin in the Senate. But under Senate rules, it takes 60 votes to end debate, or stop a filibuster, on a bill and move to a final vote.

That 60-vote bar is hard for a majority party to reach if it has only 51 members. Not only does it have to persuade at least nine Republicans to come over to its side, it also has to stop the defection of its own members.

This dynamic likely will come into play with the Employee Free Choice Act, which would permit the formation of a union if a majority of workers sign cards authorizing collective bargaining. The bill was easily approved by the House because under that chamber’s rules the majority has much more power.

But on the other side of Capitol Hill, Senate Minority Leader Mitch McConnell, R-Kentucky, says he intends to stop the so-called card-check bill. It’s likely that the Senate could be headed toward a filibuster.

Sen. Tom Harkin, D-Iowa and a strong supporter of the unionization measure, said in a conference call this week that the prospects for the bill in the Senate “are pretty good.” He meant that it could register a majority on a straight vote. “It might even garner some Republican votes,” he said.

But he admitted that getting to 60 is a different story. “That might be problematic,” he said. This situation may come up often on other bills, too.

So, as you parse the election horse race, keep an eye on Senate candidates. Watch how the GOP is faring in critical races. And then do some math. If polls are showing that Republicans are losing seats, figure out how close to 40 they’re falling. As they near that “magic” number, it becomes harder for them to stop legislation.

Even a few votes above 40 can mean a lot. In 1993, President Clinton was sworn in and the Democrats had majorities in the House and Senate. But Senate Minority Leader Bob Dole and 42 other GOP colleagues were able to exercise significant influence on the legislative agenda.

And, of course, there were times when enough Democrats from the majority turned on Clinton to defeat his proposals—for instance, the demise of the 1994 version of universal health care promoted by then-first lady Hillary Rodham Clinton.

But still, if the Republicans slide toward 40 in the Senate, it could create a big challenge for McConnell and a whole new legislative ball game on workforce issues.