Limited recent public polling has shown Cornyn with a single-digit lead over Hegar, but Democrats have been hopeful about the race as some polling also shows Trump lagging his 2016 standing in the fast-changing state.

The first ad accuses Cornyn of lying about his health care record, focusing on his votes to repeal the Affordable Care Act. SMP is also releasing a Spanish-language version of the ad.

“Three weeks out, and John Cornyn has a weaker standing than Ted Cruz ever did,” J.B. Poersch, President of Senate Majority PAC, said in a statement, adding that Hegar is running "a strong campaign."

National Democrats have been hesitant to make a full commitment to boosting Hegar, given the expense of advertising in Texas, a state that includes two of the 10 biggest TV markets in the country. The Democratic Senatorial Campaign Committee has allocated some of its coordinated funds to run ads with Hegar but hasn't made an independent expenditure in the state.

Democrats haven't won a Senate election in Texas since 1988, when then-Sen. Lloyd Bentsen won reelection — even as the ticket of Bentsen and presidential nominee Michael Dukakis lost the state to favorite son George H.W. Bush.

Hegar raised nearly $14 million in the last three months, and had $8.5 million on hand as of Sept. 30, totally erasing Cornyn's long-held cash advantage in the state. Cornyn's campaign said Thursday he raised $7.2 million in the third quarter and had $8 million in cash on hand.

Cornyn has narrowly outspent Hegar on television since mid-July, when she won the Democratic primary runoff. He has spent $9.9 million on TV and digital advertising, according to Advertising Analytics, compared to $7.4 million for Hegar. He has nearly $5 million booked through Election Day, including party-coordinated spending, while Hegar has $2.3 million, including coordinated spending.

"It’s clear that MJ’s campaign strategy is win at all costs — even if that means selling out Texas values to out-of-state liberal donors and abandoning her own promises to reject dark money," said Krista Piferrer, a spokesperson for Cornyn.

A super PAC boosting Cornyn, Texans for a Conservative Majority, is also set to spend $3.3 million in the race. The group launched its first ad hitting Hegar for taking out-of-state money.