Hillary Clinton's campaign thinks it has a chance in Arizona, a state that only two Democratic presidential candidates have won since 1948.

Clinton's camp announced Monday that first lady Michelle Obama — possibly its most effective surrogate on the campaign trail — will campaign in the state this week. Her visit will follow trips by Sen. Bernie Sanders, a popular figure among the far left and young Democrats, and Clinton's daughter, Chelsea, to Arizona.

Campaign manager Robby Mook told reporters Monday that Clinton's campaign will spend an additional $2 million in Arizona, according to NBC News. But it does not just sense vulnerability for Republicans in Arizona — Mook said it will also funnel an additional $1 million into the solid red states of Missouri and Indiana, both of which have a Senate seat up for grabs in a tight race.

Meanwhile, a challenge from independent conservative Evan McMullin in Utah has Republican presidential candidate Trump at risk of losing that solidly red state as well.

Trump could very well win all of those states, and most recent polls show him with a solid advantage in both Missouri and Indiana, where his running mate Mike Pence serves as governor. But the Clinton campaign's resource shifts show just how much vulnerability it senses for Republicans in the presidential and down ballot races, even in solidly red states.

Trump has endured a brutal two weeks in which he faced sexual assault allegations, lashed out at Republican Party leaders and trailed Clinton in almost all recent polls nationally and in crucial states Florida, North Carolina, Pennsylvania and Virginia. Trump has fared better in Ohio, where a recent NBC News/Wall Street Journal/Marist poll showed him with a 1-point advantage.

Trump has vehemently denied sexual assault allegations and repeatedly attacked both the Clinton campaign and media outlets, claiming they are trying to rig the election against him.

Against that backdrop, Clinton's campaign has sensed vulnerability in places Republicans almost always win. An Emerson poll taken even before the release of a 2005 video that shows Trump boasting about groping women without consent showed Clinton with a 2-point lead in Arizona, though Trump had led most polls before that.

Former President Bill Clinton most recently won Arizona as a Democrat in 1996. Before that, former President Harry Truman was the last Democrat to win the state in 1948.

Separately, recent Utah polls have shown Trump in a tight race with Clinton and McMullin, a conservative Mormon. If Trump does not win the state, he would become the first Republican to fail to do so since 1964.

Trump should have a much easier time holding on to Indiana and Missouri. An average of recent polls show him with a 4.5-point and 8-point lead in Indiana and Missouri, respectively, according to RealClearPolitics.

But the Clinton campaign's cash in those states may have more effect on close Senate races, where Republican candidates have had to walk a tight rope between distancing themselves from Trump and holding on to his fervent supporters. Republican Sen. Roy Blunt aims to hang on to his seat in Missouri, while non-incumbents Republican Todd Young and Democrat Evan Bayh fight for the seat in Indiana.

Mook also said Monday that Clinton will put more than $6 million into paid media and other efforts in Ohio, Florida, Pennsylvania, Nevada, Iowa and New Hampshire, according to Politico. All of those states have active Senate races. Democrats have a solid chance of unseating incumbent Republicans in at least Pennsylvania and New Hampshire.

Some election commentators have also indicated that the solidly red state of Georgia could be vulnerable, but Clinton has not led in any recent polls there.

