WASHINGTON - By most accounts, Roberta Jacobson's confirmation as U.S. ambassador to Mexico should have been a shoo-in.

Fluent in Spanish, expert in Latin American politics, and skilled in cross-border trade negotiations, the career diplomat was nominated six months ago by President Obama to take over the crucial foreign service post.

After working on Latin American affairs for both Democratic and Republican administrations for three decades, Jacobson has broad bipartisan support in Congress.

Mexico expressed enthusiastic approval and prepared to welcome her to Mexico City. The Republican-led Senate Foreign Relations Committee approved the nomination and sent it to the full Senate.

But the nomination is in limbo, hostage to GOP presidential candidate Marco Rubio's staunch opposition to Obama's diplomatic opening with Cuba, which Jacobson helped negotiate as assistant secretary of state.

Rubio, a senator from Florida, placed a hold on Jacobson's nomination in October, a legislative maneuver that blocks a confirmation vote.

"We need an ambassador in Mexico City that has the trust of Congress for this important post," Rubio explained. "I do not believe that Ms. Jacobson is that person, and will oppose her confirmation."

He cited several concerns, including the Obama administration's failure to seek timely extradition of notorious Mexican drug lord Joaquin "El Chapo" Guzman before he escaped in July from a Mexican prison.

But Rubio's sharpest knife was whetted on Cuba.

Jacobson's sin, in the senator's view, was her role in executing the rapprochement with the island's Communist-led government following Obama's decision last December to renew diplomatic ties after more than half a century of official hostility.

Jacobson subsequently led negotiations with the government of President Raul Castro aimed at opening a U.S. Embassy in Havana over the summer, easing restrictions on travel and business for Americans, and, most recently, establishing mail service between the two countries.

Rubio, the son of Cuban immigrants, accused Jacobson and the White House of failing to ensure that Cuba improve human rights before restoring ties, and of glossing over the Castro government's penchant for stifling dissent.

Though Jacobson, 55, was not the architect of the administration's Cuba policy, she was its most visible shepherd.

In an interview, she declined to discuss the nomination process, but she lamented leaving the ambassador's post vacant at a critical time.

"There are huge opportunities for Americans" thanks to structural economic reforms in Mexico, especially in the energy and telecommunications industries, she said.

"The advocacy, support, and visibility of a U.S. ambassador to help promote American businesses ... makes a difference," Jacobson said.

Rubio declined to comment for this article But his enmity for Jacobson is not new.

In 2011, an aide to then-Secretary of State Hillary Clinton expressed concern that Rubio would try to block Jacobson's promotion to assistant secretary of state, her current position, according to emails recently released by the State Department.

Rubio's focus at the time was the plight of Alan Gross, a U.S. government contractor from Maryland, who was imprisoned in Cuba. "He alludes," the aide wrote, "that he will take it out on Roberta in the confirmation process."

After five years in a Cuban jail, Gross was released last December as part of the diplomatic opening that Rubio opposed.

Any senator can slow or hold a nomination - a dozen ambassadorial nominations are pending in the Senate - but the Mexico City job is more significant than most.

The holdup means the United States has not had an ambassador in its third-biggest trading partner since August, when Ambassador Tony Wayne retired. Mexico is a multimillion-dollar export market for California and other individual states.

It also is the permanent home to about one million U.S. citizens, and 1.5 million visit on any given day, according to the State Department.

The two countries share a 2,000-mile border and are partners in numerous security agreements involving extradition, weapons trafficking, and cross-border police training.

Leaving the top U.S. diplomatic post vacant in Mexico City undermines Washington's ability to conduct international relations in this hemisphere and beyond, experts on Latin America say.

"The failure to complete her nomination sends a bad signal to our Mexican partners," said Eric Olson, associate director of the Latin American program at the nonpartisan Wilson Center in Washington, and to "all those Americans whose livelihoods and well being depend on maintaining a good and balanced relationship between neighbors."

Rep. Eliot L. Engel (D., N.Y.), ranking member of the House Foreign Affairs Committee, said that he had worked with Jacobson and that "there is no one more qualified to serve as our top diplomat" in Mexico.

"At a time when the United States faces threats all over the world," Engel said, "Congress should deepen our strong security partnership with Mexico, not take it for granted."

Rep. Linda T. Sanchez (D., Calif.) also emphasized the U.S. relationship with Mexico, citing the 22.3 million U.S.-born Mexican Americans who reside here.

"While I understand Sen. Rubio has his own political ambitions to serve," she said, "there is absolutely no justification for holding up the nomination of [such] a qualified woman."

Sanchez and 18 other Latino members of Congress wrote a letter to Rubio this month urging him to lift his "misguided" hold on the Jacobson nomination.

"Strong leadership at our embassy in Mexico City," they wrote, "has perhaps never been more important."