Democratic candidate says Republican rival “unprepared” to be president and “temperamentally unfit” to hold the office.
Republican White House hopeful Donald Trump is expected to name Indiana Governor Mike Pence as his running mate on Friday, Roll Call news site reported.
An anonymous source said Trump is going to select Pence, 57, a former US congressman, as his vice presidential nominee, Roll Call, a Washington-based organisation that reports on the politics of Capitol Hill, reported on Thursday.
Al Jazeera could not independently confirm the report.
The presumptive Republican presidential nominee will officially announce his choice on Friday at 11am (15:00 GMT) in Manhattan.
Trump is to be formally nominated as the party’s candidate for the November 8 election at the Republican National Convention next week in Cleveland. Traditionally, the vice presidential choice is used to build enthusiasm among party loyalists.
Al Jazeera’s Kimberly Halkett, reporting from Washington DC, said: “All signs indicate that it will be the Indiana governor, Mike Pence. He has an awful lot of experience that Donald Trump doesn’t have.”
Our correspondent added that Trump, a former reality television star and businessman, does not have the governing or legislative experience that Pence possesses.
“Pence has served in congress for six terms. That complements Donald Trump’s resume, making him more appealing to voters,” she said.
Trump’s choice of running mate is seen as critical because his defeat of 16 rivals in the Republican primary race left the party divided. Some party leaders are still uneasy about some of his campaign positions, and his style.
Roll Call said Trump was reportedly impressed with Pence’s calm demeanour, his experience on Capitol Hill and as a governor, and Pence’s potential to assist in governing if Trump wins in November. Trump, a New York businessman, has never held elected office.
Trump had also considered former House of Representatives Speaker Newt Gingrich and New Jersey Governor Chris Christie as finalists.
Gingrich told an ABC News correspondent earlier that he expected to hear Trump’s decision soon and would not be surprised if Pence was selected.
Pence is seen as a safe choice, not too flashy but popular among conservatives, with Midwestern appeal and the ability to rally more party faithful behind Trump.
Pence had backed a Trump rival, Senator Ted Cruz of Texas, in April before the Indiana primary, but he praised Trump and said he would work on behalf of the eventual Republican nominee.
He is currently seeking re-election as governor of Indiana, a US midwest state. Indiana law prevents him from seeking two offices at once, and he faces a Friday deadline to withdraw from the governor’s race.
Pence and Trump spent time this month testing their chemistry at Trump’s golf course in New Jersey and at the governor’s residence in Indiana, Roll Call said.
He had considered running for president himself in 2016 before deciding to run for re-election as governor. Conservatives had urged him to seek the White House, but missteps in 2015 related to an Indiana law seen as anti-gay hurt his national profile.
This year, he was the target of a mocking social media campaign by women outraged at a law he signed which added restrictions on abortions. Feeling that the law invaded their privacy, women responded by calling Pence’s office to describe their menstrual periods or tweeting similar messages.
Pence ran unsuccessfully for Congress twice before he was elected to the House of Representatives in 2000, where he was chairman of the Republican Study Committee, a group of conservatives.
In what has been an unusually public process of making his choice of running mate, Trump, 70, met both Pence and Gingrich separately in Indianapolis on Wednesday.
He also met with a fourth potential running mate , US Senator Jeff Sessions. The 69-year-old of Alabama has been one of Trump’s closest advisers.
Trump had dinner with Pence on Tuesday night after they appeared together at a rally.
Trump adviser Ed Brookover told CNN that “first and foremost” Trump wants a running mate who he has good chemistry with and someone who can help him govern best.