Roberta Jacobson, assistant secretary of state for the Western Hemisphere and top negotiator with Cuba, has been nominated to be the new U.S. ambassador to Mexico.

President Obama announced her as part of a list of federal nominations that still require the U.S. Senate's approval.

The announcement was made on Monday, June 1 in a press release. The president nominated Assistant Sec. Jacobson, as well as three others, for various federal offices. Assistant Sec. Jacobson would replace current ambassador to Mexico, E. Anthony Wayne, who has held his position since 2011.

The White House has yet to announce who will replace Assistant Sec. Jacobson, but her successor would continue the task of reestablishing diplomatic ties with Cuba. The assistant secretary has been negotiating with Cuban officials for months to reestablish formal relations after half a century.

The two nations have started addressing the reopening of embassies. The announcement could be coming in the next few weeks.

"We have made significant progress in the last five months and are much closer to reestablishing relations and reopening embassies," she said in a press conference.

Assistant Sec. Jacobson has been in this position since 2012. Her office "is responsible for managing and promoting U.S. interests in the region by supporting democracy, trade, and sustainable economic development, and fostering cooperation on issues such as drug trafficking and crime, poverty reduction, and environmental protection," according to the State Department.

Her position and experience would help her as ambassador to Mexico. However, Mexico may prove to be a challenge. The nation has been embroiled in a quagmire of violence for more than a decade, battling drug cartels and other criminals as well as corruption. The struggle continues to be deadly, with a recent conflict between police and drug traffickers ending with two dead.

However, one person will stand between Assistant Sec. Jacobson and Mexico City: Sen. Marco Rubio. The Florida Republican penned an open letter on Monday addressed to Secretary of State John Kerry vowing to not approve any ambassadorial nominations until the issue of Cuba is resolved.

"Despite more than five months of discussions with the Cuban government, I am very concerned about the lack of political reforms, and progress on human rights; the continued harboring of known terrorists and other fugitives from U.S. justice; the outstanding American property claims and judgments against the Cuban government; and the limitations that continue to be placed on American diplomats working in Havana," he wrote.

Although he did not address Assistant Sec. Jacobson's nomination directly, the senator's letter addressed her negotiations with Cuba, arguing the administration has not done enough to address the issues of human rights, fugitives, and monetary compensation.

Sen. Rubio wants the Obama administration to ensure Cuba returns criminals who fled there, including Joanne Chesimard, who murdered a New Jersey police officer. He also demands repayment for American properties seized during the Cuba Revolution and the free movement of embassy employees.

On the latter, the assistant secretary has mentioned her team's continued effort to ensure American embassy workers have the same freedoms as others elsewhere.

In a Republican-controlled Senate, it will be hard for Assistant Sec. Jacobson to find the majority votes need for confirmation.