Senate Democrats have maintained a united front as a minority in the new Congress — so far.

No Democrats crossed party lines on the Homeland Security funding bill vote. Last week’s vote on the Keystone XL pipeline saw some defections. But President Obama’s veto power can easily withstand a few Democrats siding with the GOP. There was no damage to Senate Democrats’ unity.

Now comes a new and very dark cloud.

Votes on potentially large U.S. trade deals hold the potential for a storm that will scatter the band of brothers and sisters known as Senate Democrats.

The same votes also threaten to alienate them from Obama for his last two years.

The cloud will become a self-destructive storm for the Democrats if their left-wing faction, led by Sen. Elizabeth Warren, D-Mass., joins with right-wing Republicans.

The Tea Party element reflexively wants to block any bill that could one day be counted as an achievement for Obama.

The popular Sen. Warren is framing her concern over the proposed trade pacts by pointing to past deals that she contends pushed “America’s middle class in a deep hole,” while boosting big business and Wall Street investors.

The Tea Party element reflexively wants to block any bill that could one day be counted as an achievement for Obama.

On the other hand, Senate Democrats can find motivation for supporting the president by looking at a June Pew poll. It found 59 percent of Americans backing free trade deals such as the Trans-Pacific Partnership.

The Tea Party element reflexively wants to block any bill that could one day be counted as an achievement for Obama.

More motivation comes from history. President Bill Clinton won approval of the 1994 North American Free Trade Agreement (NAFTA) from a Republican Congress. In 2002, it was a Republican majority that again gave “fast track” power on trade deals to the Bush White House. Congressional Democrats have to be concerned about being left behind one more time on this critical issue.

In this go-around, Democrats on Capitol Hill risk looking less like faithful defenders of the working class than bystanders to the changing reality of global trade.

Obama is hoping he can pull the majority of Senate Democrats to his side by making precisely this argument.

“Those who oppose these trade deals ironically are accepting a status quo that is more damaging to American workers,” the president told the Business Roundtable in December. He cautioned Democrats to not “fight the last war” over problems with lost jobs due to the NAFTA deal.

“There are folks in my own party… that have legitimate complaints about some of the trend lines of inequality, but are barking up the wrong tree when it comes to opposing [the Trans-Pacific Partnership],” the president said.

“Globalization is a force,” Michael Froman, President Obama’s U.S. Trade Representative, recently told John Harwood of The New York Times. Froman, who is leading negotiations with Democrats in Congress, acknowledges problems with past trade deals as well as middle-class anxiety about competing against low-pay workers overseas. But Froman, like Obama, insists new markets for American products will lead to increased exports, more jobs and high wages.

Globalization is “out there because we have container ships, because we have air travel, because we have broadband,” Froman told The Times, echoing his words in meetings with Democrats. “That’s made it easier for companies to locate at different places around the world. Trade agreements are how we shape globalization.”

Despite the power of that logic, doubts persist among Democrats. When Democrats held the Senate majority they stonewalled Obama on trade deals. Then-Senate Majority Leader Harry Reid, D-Nev., said he could not sell “fast track” powers in the face of opposition from labor unions and environmental groups, key elements of the Democratic constituency.

Already, 150 of the 188 House Democrats have signed letters of opposition to giving the president an up-or-down vote on so-called “Fast Track” authority on negotiations for a 12-nation deal. This is the Trans-Pacific Partnership. The nations that would be included under such a deal would account for 40 percent of the global economy. But in January Rep. Rosa DeLauro, D-Conn., branded “Fast Track” as an “insult to the American worker” and promised there was no way her fellow Democrats would support it. “We are not going to do it.”

Among Republicans, opposition to the trade deal is based in their long-standing opposition to any proposal from Obama.

There is also Tea Party distaste for ceding U.S. sovereignty to an international group governing the trade deal. But Republicans look like they have the votes to pass the bill in the House and Senate. They are helped by two things: fear of Chinese domination of Asian trade and overwhelming pressure from the U.S. Chamber of Commerce, the Business Roundtable and Wall Street.

House Speaker John Boehner, R-Ohio, and Rep. Paul Ryan, R-Wis., chairman of the House Ways and Means Committee, have promised to deliver the votes. Ryan calls increased trade the key to growing good-paying jobs in the future.

“I agree with every word [in Obama’s State of the Union address] with respect to trade and Asia and getting in there and helping write the rules instead of China writing the rules,” Ryan told reporters, adding that the deal means “more jobs for America.”

This is one time when Senate Democrats might show strength and unity by listening to Paul Ryan.

Juan Williams is a co-host of FNC's "The Five," where he is one of seven rotating Fox personalities.