President Biden is scrambling to avert a series of looming strikes that could force him to choose between his promise to be the most pro-union president in American history and the politics of a presidential campaign season.

Last week, the Teamsters union issued a strike notice against financially strapped trucking giant Yellow Corp. and signaled that the work stoppage could start as soon as Monday. Next in line: shipper UPS, which handles one-fourth of America’s parcel shipments.

The Teamsters’ combative president, Sean O’Brien, said at an Atlanta rally Saturday that union drivers would stop driving the big brown trucks as soon as August 1 unless a deal is made by a July 31 contract deadline. Mr. O’Brien promised at the rally to “pulverize” UPS and cripple a vital cog in the nation’s supply chain. Union members have already been marching in “practice” pickets in cities across the country over the weekend.

Not far behind the Teamsters is the United Auto Workers union, whose new president, Shawn Fain, met with Mr. Biden at the White House last week. The union has sharply criticized the president in the past for funneling billions in taxpayer subsidies to battery makers, electric vehicles, and other green energy projects in union-averse right-to-work states and has been one of the few unions to withhold its endorsement of the incumbent as a result.

The autoworkers are currently negotiating a new contract to replace one that ends in September with the big-three automakers, and Democrats in Washington are already warning of the prospect of strikes that would dampen enthusiasm for the president in the critical battleground state of Michigan. President Trump, in a long-shot bid to snatch the union’s endorsement, has joined the chorus of criticism of what he referred to in a video released last week as Mr. Biden’s “ridiculous green new deal crusade.”

“I hope United Auto Workers is listening to this because I think you’d better endorse Trump,” Mr. Trump said in the video. Mr. Fain has sought to disabuse Mr. Trump of that prospect, telling union members in a recent memo that a second Trump presidency would be a “disaster” for the country.

Mr. Biden tried to strike an optimistic tone during a speech at a Pennsylvania shipyard last week, insisting that the administration’s subsidies for offshore wind projects and other green energy initiatives are creating good-paying jobs across the United States. “We’re making sure these new jobs come free and fair and [with] the ability to join a union if you’re not already in a union,” the president told a group of largely union observers.

The reality, however, is that most of the jobs created so far under Mr. Biden’s tenure have gone not to Rust Belt states in the Midwest but rather to largely non-union red states in the South such as Florida, Georgia, and Texas. California, which has added 79,000 manufacturing jobs since Mr. Biden took office, according to Bureau of Labor Statistics data cited by Axios, is the only blue state to have gained ground so far.

The disconnect has not gone unnoticed by unions like the United Auto Workers and the Teamsters, both of whom have demonstrated a willingness to play hardball with both the companies that employ their members and the politicians who allegedly support them. Messrs’ O’Brien and Fain, both of whom are newcomers to their jobs, have struck a decidedly more militant tone than their predecessors, one that has reportedly caught the Biden administration off-guard.

Both unions have asked the White House not to intervene in any strikes, an apparent effort to avert a replay of last year’s threatened strike at the railroads. Mr. Biden infuriated his union backers by forcing an agreement on both sides at the time, and he will likely be tempted to do so again if simultaneous work stoppages occur at pivotal moments in the coming presidential campaign.