MADISON, Wis. — Two groups that are part of the network run by conservative Kansas billionaire Charles Koch unleashed $1.6 million in new ads attacking Democratic Sen. Tammy Baldwin on Wednesday, raising spending in a race that's already one of the most expensive in the country.

The spots came a day before Vice President Mike Pence was to host a Thursday night fundraiser in Milwaukee for Baldwin's Republican challenger, state Sen. Leah Vukmir, a move to help her keep pace with the better-funded Baldwin. Pence praised Vukmir in a radio interview ahead of the event, promising ongoing support from both him and President Donald Trump in the closely watched race.

Also Wednesday, Baldwin introduced a resolution co-sponsored by 29 other Democrats to force a vote on overturning a Trump administration rule allowing the purchase of short-term health plans critics have called "junk insurance."

The new television and digital ads come from Concerned Veterans for America and Americans for Prosperity-Wisconsin. Each said it was spending $800,000 on the spots. One ad features veterans accusing Baldwin of missing more than 100 "important hearings" of the Senate Committee on Homeland Security and Governmental Affairs.

Baldwin's campaign has provided records showing that she has a 100 percent voting record on the committee. Senators do not need to be physically present to have a vote recorded.

The other ad said there's a "clear choice" on taxes and spending, touting Vukmir's votes in the state Senate to cut taxes and pledging to oppose raising taxes.

The new ad spending brings the total the two Koch-funded groups have spent in the race against Baldwin to more than $6 million. The Center for Responsive Politics says more than $19 million has already been spent by outside groups in the Wisconsin Senate race, helping to make it the third-most expensive contest in the country.

Baldwin's campaign spokesman Bill Neidhardt said the Koch groups were targeting Baldwin because they know Vukmir, a national board member on the conservative American Legislative Exchange Council, "will do their bidding."

"Wisconsinites want someone in their corner and they know Tammy Baldwin stands up to these corporate special interests," Neidhardt said.

Baldwin's resolution introduced Wednesday would force a vote to rescind a rule allowing people to buy short-term insurance plans that are cheaper than what is available under the Affordable Care Act but have fewer benefits.

Supporters argues the expanded options are necessary, while Baldwin and other critics have said they are "junk" because they can deny coverage to people with pre-existing conditions and don't cover essential services like prescription drugs, emergency room visits and maternity care.

Republican Sen. Ron Johnson criticized the move, saying Baldwin was trying to "restrict the options families have to choose the health insurance that is right for them."

"Sen. Baldwin continues to ignore the forgotten men and women who, under Obamacare, have seen their premiums double, triple, and in some cases quadruple," Johnson said.

Baldwin's proposal would have to be approved in both the House and Senate to overturn the rule, something almost certain not to happen under Republican control.

Baldwin has tried to make health care a major focus of her re-election bid against Vukmir, a retired nurse who has also made it a central part of her campaign. Health care was pegged as the single biggest issue in the Senate race in a Suffolk University poll conducted for the Milwaukee Journal Sentinel and released Tuesday.