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 by President Obama to take over the crucial foreign service post six months ago.
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.
The hold-up means the United States has not had an ambassador in its third-biggest trading partner since August, when Ambassador Tony Wayne retired.
Mexico is an important export market for California and other states. It also is the permanent residence of approximately 1 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.
"The failure to complete her nomination sends a bad signal to our Mexican partners and all those Americans whose livelihoods and well being depend on maintaining a good and balanced relationship between neighbors," said Eric Olson, associate director of the Latin American program at the nonpartisan Wilson Center in Washington.
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 from a Mexican prison in July.
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 last 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 story. 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 that he will take it out on Roberta in the confirmation process," the aide wrote.
After five years in a Cuban jail, Gross was released in December 2014 as part of the diplomatic opening that Rubio opposed.
Any senator can slow or hold a nomination -- a dozen ambassadorial nominations are currently pending in the Senate -- but the Mexico City job is more significant than most.
Rep. Eliot L. Engel (D-NY), ranking member of the House Foreign Affairs Committee, said he had worked with Jacobson and "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, Congress should deepen our strong security partnership with Mexico, not take it for granted," Engel said.
Rep. Linda T. Sanchez (D-Whittier) 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, there is absolutely no justification for holding up the nomination of [such] a qualified woman," Sanchez said.
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 has perhaps never been more important," they wrote.