Sen. Russ Feingold (D-Wis.) faces a millionaire opponent in Republican Tim Michels, but it is debatable whether he will be able to take advantage of a provision in the campaign finance law he authored to help candidates of modest means.

The “millionaires amendment” of the Bipartisan Campaign Reform Act is triggered when wealthy candidates contribute a set amount of their own money or more to their campaigns.

That amendment, which Feingold and Sen. John McCain (R-Ariz.) did not include in their original bill, allows less-affluent candidates to accept contributions above the normal amounts allowed by law if their wealthy opponents have reached a threshold of self-financing.

In some scenarios, the caps on “coordinated” spending by party committees could be lifted entirely.

Unfortunately for Feingold, it appears that Michels would have to contribute possibly as much as $4 million of his own money before Feingold could take advantage of the higher contribution limits.

That is because the law takes into account situations where a less well-off opponent may prove a prolific fundraiser and be at no financial disadvantage at all, regardless of his opponent’s wealth.

That seems to be the case in Wisconsin.

The complicated law sets out intricate formulas for calculating when the wealthy candidate contributes that “threshold” amount and when the poorer candidate can benefit.

Feingold had raised around $9 million since his last election while Michels had raised about $2.4 million, according to the Aug. 25 Federal Election Commission reports.

When the roughly $1.25 million Michels has given or loaned his campaign is discounted, as the law requires, the difference in fundraising between Feingold and Michels is vast.

Michels will easily trip the $314,000 initial threshold amount, but his campaign has not said how much more he would contribute beyond that.

Michels is reluctant to commit a set amount of his money to the race until he figures out the implications of the finance laws, said Tim Roby, his campaign spokesman.

Michels’ team is still trying to determine what impact his millions would have.

In the meantime, he is focusing on fundraising.

The more money Michels can get from other people, the less likely it is that Feingold could benefit from the millionaires amendment, Roby said.

The Feingold camp has also adopted a wait-and-see approach.

“Our position is if and when that hypothetical happens, he will decide at that time” what to do, said John Kraus, Feingold’s campaign spokesman.

Feingold has not ruled out seeking contributions above the $2,000 individual limit if he is allowed to, but he also has not decided that he definitely would either, Kraus said.

If Michels can spend $3 million or $4 million of his own money without giving Feingold an advantage, the already closely watched race could tighten considerably.

Feingold went into the general election with about $3 million left in his war chest.

Michels, who just won a three-way primary Sept. 14, begins from almost zero, having spent more than $2 million, about $1.5 million of it his own, in the primary. But he need only turn to his own bank account for replenishment, whereas Feingold must tap donors to bolster his campaign account.

Kraus says the Feingold camp did not make a judgment error by already spending almost $7 million since 1999, despite not having a primary opponent.

“We’ve had three opponents running a ‘bash-athon’ on us for a year,” Kraus explained. “We weren’t going to sit on the sidelines.”

The Republican Senate candidates attacked Feingold as much as each other during their primary contest.

As a result, Feingold has been on television almost nonstop since June.

“We’ve run against millionaires before — this isn’t anything we haven’t seen before and beat,” Kraus said.