David Jackson

USA TODAY

While formally nominating retired Marine general John Kelly to be secretary of Homeland Security, Donald Trump and aides spent Monday interviewing other job applicants and disputing CIA claims that Russia interfered in the election to help elect the New York businessman.

"Unless you catch 'hackers' in the act, it is very hard to determine who was doing the hacking," Trump tweeted, echoing comments he made on Fox News Sunday over the weekend.

In another tweet, Trump said: "Can you imagine if the election results were the opposite and WE tried to play the Russia/CIA card. It would be called conspiracy theory!"

Trump and his aides said Democrats are pushing the Russia angle in order to excuse their election loss. Trump transition spokesman Jason Miller called it part of an effort to "de-legitimize" the president-elect's victory over Democrat Hillary Clinton.

Senate Majority Leader Mitch McConnell, R-Ky., meanwhile, joined a growing list of lawmakers from both parties in calling for a probe into Russia's election activity, including email hacks of Democratic and Clinton campaign officials.

"Obviously any foreign breach of our cybersecurity measures is disturbing, and I strongly condemn any such efforts," the Republican leader told reporters.

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On social media, Trump asked, "why wasn't this brought up before election?" But allegations of Russian interference surfaced frequently during the fall campaign, even during the debates between Trump and Clinton. "Will Donald Trump admit and condemn that the Russians are doing this and make it clear that he will not have the help of Putin in in this election?" Clinton said during the third debate.

The issue amped up over the weekend amid news reports that CIA officials believe Russia interfered with the specific goal of aiding Trump's election.

On another security front, Trump said he is nominating Kelly to be secretary of Homeland Security in order to combat terrorism and illegal immigration.

"Gen. John Kelly’s decades of military service and deep commitment to fighting the threat of terrorism inside our borders makes him the ideal choice to serve as our Secretary of the Department of Homeland Security,” Trump said in a statement. “He is the right person to spearhead the urgent mission of stopping illegal immigration and securing our borders, streamlining TSA and improving coordination between our intelligence and law enforcement agencies."

Accepting the appointment, Kelly said that "the American people voted in this election to stop terrorism, take back sovereignty at our borders, and put a stop to political correctness that for too long has dictated our approach to national security.

"I will tackle those issues with a seriousness of purpose and a deep respect for our laws and Constitution," Kelly said.

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In other transition developments:

• Trump confirmed his appointment of Goldman Sachs executive Gary Cohn to be director of the National Economic Council.

"As my top economic adviser, Gary Cohn is going to put his talents as a highly successful businessman to work for the American people,” Trump said in a statement. "He will help craft economic policies that will grow wages for our workers, stop the exodus of jobs overseas and create many great new opportunities for Americans who have been struggling."

Calling the appointment a great honor, Cohn said he shares Trump's "vision of making sure every American worker has a secure place in a thriving economy, and we will be completely committed to building a nation of strength, growth and prosperity.”

• China said it has "serious concern" about Trump's comments that he may change the "one China" policy in favor of a different relationship with Taiwan.

"Upholding the 'one China' principle is the political basis for developing China-U.S. ties," China Foreign Ministry spokesman Geng Shuang told reporters. "If this basis is interfered with or damaged then the healthy development of China-U.S. relations and bilateral cooperation in important areas is out of the question."

• Trump's list of interviews Monday included a pair of former primary rivals who are candidates for jobs in his administration.

The president-elect spoke with former Texas governor Rick Perry, a possible nominee for Energy secretary, and businesswoman Carly Fiorina, a possible nominee for Director of National Intelligence.

After her interview, Fiorina told reporters: "We talked about hacking, whether it’s Chinese hacking or purported Russian hacking."

News of Kelly's appointment had surfaced last week.

A native of Boston, Kelly first enlisted in the Marine Corps in 1970, and rose to become commander of the U.S. Southern Command, supervising military operations in Central and South America and the Caribbean. He is also a Gold Star father who lost his son during combat in Afghanistan in 2010.

The Department of Homeland Security, created in the wake of the 9/11 attacks, oversees a variety of security agencies, including Customs and Border Protection, Citizenship and Immigration Services, the Coast Guard, the Federal Emergency Management Agency, Immigration and Customs Enforcement, the Secret Service, and the Transportation Security Administration.

Kelly is the third retired general to join Trump's national security team, along with national security adviser Michael Flynn and Defense secretary nominee James Mattis.

Kelly and Mattis must both be confirmed by the Senate. Mattis needs a special waiver from Congress to serve, because he has not been retired from the military for at least seven years.

In its announcement, the Trump transition team noted that Kelly's years as head of the Southern Command gave him "unique insight into some of the challenges the United States faces at its southern border."

The Trump team also said Kelly "was intimately involved in, and often coordinated, the whole of U.S. Government interagency efforts in the interdiction of drug and human trafficking, counter-terrorism, human rights, and working with partners to safeguard southern approaches to the U.S. border."