The Associated Press

Warren Buffett, a long-time Democrat who supported Hillary Clinton, said he will judge Donald Trump’s presidency four years from now based on how safe he has kept the country, how the economy is doing and on how many people are sharing in the wealth.

The billionaire investor told CNBC on Monday that the No. 1 job of the chief executive of the United States is how safe the country has been kept and "that's not an easy job." His number one worry is weapons of mass destruction and says that is the biggest test for the president.

He says he will then judge him on "if the economy does well, which I expect it to do, how wide the participation in a better economy extends."

When asked if Trump passes on all three of those issues would you consider voting for him in four years, Buffett said: "Well it depends on who he is running against. I would say it would be unlikely."

Buffett says it is a mistake to mix up political decisions with investing ones and that he agrees with President Donald Trump on some issues. He generally agrees with what Trump says about entitlement programs and Social Security, and some industries do face too many regulations.

Buffett appeared on CNBC after releasing his annual letter to Berkshire Hathaway shareholders on Saturday.

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Here is what Buffett said on other issues:

Major tax policy overhaul unlikely

Buffett says the border adjustment tax proposal Republicans are considering would effectively increase the price of imports.

Buffett says if retailers like the Berkshire Hathaway-owned Nebraska Furniture Mart has to pay more taxes on imports, that will increase the price consumers pay.

But Buffett says he thinks it’s likely Republicans will drop the border-adjustment tax proposal because it would be difficult to pass such a dramatic change in tax policy quickly.

Buffett says he has a feeling the tax changes Republicans try to pass this year won’t be a major overhaul because they are trying to move quickly. Plus, he says lobbyists from every industry will oppose major changes.

Stock market is not in bubble territory

Despite record high levels hit by the Dow Jones and S&P 500 this year, the billionaire believes stock prices are cheap, when considering the current low interest rates.

He revealed he's ploughed $20 billion into the market over the last four months.

The chairman and chief executive of Berkshire Hathaway told CNBC: "We are not in a bubble territory."

"If rates were to spike, however, then the stock market would be more expensive."

Buffett also said he's more than doubled his holdings in techonology giant Apple since the new year.

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Airline stocks

Buffett says his company bought more than $9 billion of airline stocks because he believes they are operating more efficiently than they used to.

Buffett has long recommended that investors avoid airline investments because they required significant capital and delivered poor returns.

After all the bankruptcies and consolidation in the industry, Buffett says the airlines are performing better and generally flying with their planes at least 80% full.

Buffett says airlines are “a business you can always mess up.” But Buffett says he thinks airlines are performing better after “a bad century.”

Berkshire Hathaway holds sizeable stakes in American Airlines, Delta Air Lines, United Continental and Southwest.

At the end of 2016, Berkshire held 45.5 million shares of American, 60 million shares of Delta, 43.2 million shares of Southwest and nearly 29 million shares of United Continental.

Unilever bid

Buffett says the proposed $143 billion offer to acquire Unilever, a deal with links to his company, fell apart quickly because the European company wasn’t interested.

Buffett said Monday on CNBC that his Berkshire Hathaway and 3G Capital abandoned Kraft Heinz Foods’ bid for Unilever because they were interested only in a mutually agreed upon tie-up, not a hostile takeover.

Buffett says it initially appeared that Unilever might be open to negotiating a deal, but once a rough offer was delivered it became clear the owner of Lipton and Hellman’s wasn’t interested.

Buffett says there could have been some misunderstanding in the initial conversations between Kraft Heinz and Unilever.