Castro, 41, makes for a striking tableau standing next to Clinton: a rising Latino star of the Democratic Party, paired with the older, experienced, first female major party nominee, gives the sense of a big tent party. Castro has been working hard for Clinton on the trail, traveling to 11 states as a surrogate, but Democrats close to the campaign said there is concern his inexperience would cramp Clinton’s ability to frame Trump as someone lacking the appropriate resume for the Oval Office.

The progressive revolt against Castro over his handling of mortgage sales — no small concern for a party struggling to win over voters with deep reservations about Clinton’s brand of center-left pragmatism and her ties to Wall Street — has also raised some red flags.

Warren, 67, the fantasy number two pick for many progressive-leaning Democrats, offers the promise of a historic two-woman ticket, and would help soothe the hurt feelings of millions of Bernie Sanders supporters who find great appeal in the message of an economic system that is rigged for those at the top.

But the Massachusetts senator is seen as a potentially difficult bedfellow for the next eight years, and there is a lack of any personal relationship between the two leaders to draw on. There’s also a sense in Clinton circles that in a race against Trump, Clinton doesn’t need to make what would be a buzzy but risky bet with a number two who has her own clear agenda to push — one that has not always been in line with Clinton’s. Warren insiders have expressed distrust of Clinton on her core issue of Wall Street reform.