Republican presidential candidate Rand Paul looks at Donald Trump's past comments on Democrats in his latest ad, which he posted on YouTube. (Rand Paul)

UPDATE: At 6:50 p.m., Donald Trump sent The Washington Post a lengthy response to Kentucky Sen. Rand Paul's ad. Here it is, in full.

Rand Paul is doing so poorly in the polls he has to revert to old footage of me discussing positions I no longer hold. As a world-class businessman, who built one of the great companies with some of the most iconic real estate assets in the world, it was my obligation to my family, my company, my employees and myself to maintain a strong relationship with all politicians whether Republican or Democrat. I did that and I did that well.

Unless you are a piece of unyielding granite, over the years positions evolve as they have in my case. Ronald Reagan, as an example, was a Democrat with a liberal bent who became a conservative Republican.

Recently, Rand Paul called me and asked me to play golf. I easily beat him on the golf course and will even more easily beat him now, in the world in the politics.

Senator Paul does not mention that after trouncing him in golf I made a significant donation to the eye center with which he is affiliated.

I feel sorry for the great people of Kentucky who are being used as a back up to Senator Paul’s hopeless attempt to become President of the United States--- weak on the military, Israel, the Vets and many other issues. Senator Paul has no chance of wining the nomination and the people of Kentucky should not allow him the privilege of remaining their Senator.  Rand should save his lobbyist’s and special interest money and just go quietly home.

Rand’s campaign is a total mess, and as a matter of fact, I didn’t know he had anybody left in his campaign to make commercials who are not currently under indictment!

At 7:32 p.m., Paul campaign strategist Doug Stafford sent this response.

Wow, that took a while to read.

First, Ronald Reagan spent 20 years as a conservative before running for President, not twenty minutes. He changed out of conviction. He campaigned for Goldwater in 1964 giving one of the great conservative speeches of all time, setting the intellectual agenda for a generation of conservatives.

Donald Trump couldn't set the intellectual conservative agenda of anything, not even the tiniest rooms, never mind a country. He is devoid of ideas other than he likes the idea of power and getting attention for foolish statements and bluster.

Rand Paul is the one following in the footsteps of Reagan, setting the intellectual agenda for a conservative movement of change. Rand stands for principle. He has detailed plans to end our debt by balancing the budget in 5 years. He has a detailed flat and fair tax that would be a huge tax cut for Americans while ending the corporate welfare gravy train for people like Donald Trump. He has real plans to defeat the Washington machine like term limits and forcing Congress to read the bills.

While he appreciates Donald's golf skills, I will note that [the game] was on his home course that he plays often. And he does sincerely appreciate Donald's generosity to the eye clinic. In fact he has mentioned it often, including in his op-ed and speeches this weekend.

The fact is, Rand is running to fight the big business, big government establishment. Donald Trump already represents one end of that problem. Now he wants to represent the other. It won't work.

Trump's initial response came several hours after Rand Paul's presidential campaign released an aggressive attack video Wednesday questioning Trump's conservative bona fides.

“I probably identify more as a Democrat,” Trump is shown saying in the video. "I've been around for a long time, and it just seems that the economy does better under the Democrats than the Republicans.” The words imposed on the screen as Trump speaks: "I ... IDENTIFY MORE AS A DEMOCRAT." (The all-caps are all theirs.)

The Paul campaign said the ad would run in New Hampshire and Iowa through the weekend. "In sharp contrast, Senator Rand Paul has been a true conservative who has always stood up to the Washington machine," a campaign spokesman said in a statement to the press about the new spot.

Paul and Trump engaged in a tense back and forth during the first Republican primary debate last Thursday, in which Paul called into question Trump’s party loyalty after the real estate tycoon said he would not pledge not to run as an independent if he loses the GOP nomination. Trump fired back at Paul, insinuating that Paul was being bought by donors.

The next Republican presidential debate is Sept. 16 in Simi Valley, Calif.