WASHINGTON (Reuters) - Republican presidential hopeful Marco Rubio has no plans to allow a vote on President Barack Obama’s nominee to be ambassador to Mexico, a spokesman said on Thursday, despite calls from his fellow Republicans and Democrats in the Senate to let her confirmation move ahead. The long delay in confirming career diplomat Roberta Jacobson has contributed to concerns about the state of U.S.-Mexico relations fueled by aggressive rhetoric against its citizens in the race to be the 2016 Republican nominee. Three Democratic and one Republican U.S. senator held a news conference on Thursday calling for her nomination to move ahead. There has been no U.S. ambassador in Mexico City for nine months. Donald Trump, the front-runner for the Republican nomination, has labeled Mexicans drug runners and rapists and called for a massive wall along the border. Rubio and fellow Republican Senator Ted Cruz have also proposed building a wall. Obama nominated Jacobson in June. She has faced resistance from Rubio and some other lawmakers, however, for reasons including her role in helping negotiate Obama’s easing of relations with Communist-ruled Cuba and what they say is the administration’s failure to prioritize human rights. Rubio, a Cuban-American from Florida, has been a leading opponent of the policy, saying Havana should improve its rights record before trade and travel restrictions are eased. The Senate Foreign Relations Committee approved Jacobson’s nomination in November, with Republican and Democratic support. But Rubio has been blocking her, under a Senate rule allowing any member to “hold” a nomination indefinitely. “Senator Rubio maintains his strong opposition to this nominee on the basis of her failure to be transparent during the confirmation process, inability to provide straightforward answers, and poor track record on a wide range of issues as the assistant secretary of state for Western Hemisphere affairs,” Rubio spokesman Alex Burgos said. Senators from states that border Mexico held a news conference with the U.S. Hispanic Chamber of Commerce calling for Jacobson’s confirmation, citing the importance of the U.S.-Mexico relationship. Mexico is the second largest market for U.S. exports and the country’s third-largest trading partner. “It’s extremely important,” said Republican Senator Jeff Flake of Arizona. Democrat Tom Udall of New Mexico, who tried to bring up Jacobson’s nomination in the Senate but was blocked by Republican Mike Lee on Rubio’s behalf, said he was confident Jacobson would be confirmed if a vote went ahead. 