The former Republican presidential candidate and his vice president pick will campaign together Friday for the first time since losing the presidential race in 2008.
Palin was a first-term governor of Alaska when McCain plucked her from relative obscurity to be his running mate. She went on to become a conservative star and a key Republican critic of President Barack Obama and the Democrats in Congress.
McCain is fighting for his political life. Fending off a primary challenge from the right, the four-term Arizona senator is facing the toughest re-election campaign of his Senate career.
Former congressman and conservative talk-radio host JD Hayworth says McCain is too moderate for Arizona Republicans. He points to McCain's reputation for working with Democrats on key issues such as reducing greenhouse gas emissions and restricting campaign donations.
Palin will help McCain tout his conservative credentials at rallies in Tucson on Friday and the Phoenix suburb of Mesa on Saturday. They'll hold a fundraiser on Friday at the same Phoenix hotel where they conceded the presidential election on Nov. 4, 2008.
Hayworth has tried to define himself as "the consistent conservative" in contrast to the "maverick" McCain.
Before Hayworth left his radio show to officially enter the race, he used the airwaves to attack McCain's congressional record, most notably his work with the late Democratic Sen. Edward M. Kennedy on a bill that would have created a pathway to citizenship for illegal immigrants.
Now, Hayworth is hoping to topple one of the Republican Party's best-known figures by reaching out to conservative activists.
Hayworth said Palin is repaying McCain for launching her national political career.
"We look forward to having Gov. Palin's support following the primary," Hayworth said. "But we welcome her and we understand why she's in the state stumping for McCain."
Palin's popularity and fundraising power is largely unmatched on the right. But she's also been berated as a lightweight not prepared for national office, and she was criticized last year for resigning as Alaska governor before her term was up.
Palin has admonished McCain's presidential campaign since their loss, saying in her book "Going Rogue" that there was substantial tension between her advisers and McCain's. She said she was kept "bottled up" from reporters during the campaign and was prevented from delivering a concession speech in Phoenix on Election Night.
Palin hasn't criticized McCain himself, however, and the senator has stood by his decision to choose her as his running mate, saying he was proud of the campaign and predicting she would be a "major player" in the Republican Party.
Palin took heat this week when she released a list of 20 U.S. House of Representatives seats she said conservatives should target in the upcoming midterm elections. The list, posted on her Facebook page, featured a U.S. map with circles and cross hairs over the 20 districts.
Critics said it was inappropriate to use gun imagery, especially as a handful of Democrats who supported the health care overhaul reported receiving threats of violence.
McCain defended Palin, saying it was common practice and "part of the lexicon" to refer to targeted congressional districts.