WASHINGTON ― The Senate takes up the 2020 defense policy bill on Monday, but votes on hot topics like Pentagon support at the southern border, the president’s authority to go to war with Iran, and his continued authorities for Iraq and Afghanistan could be derailed by congressional fights.

Senators have offered roughly 600 amendments to the massive National Defense Authorization Act, which authorizes $750 billion for national defense. But in recent years, more controversial amendments have been blocked while only packages of unobjectionable amendments have passed.

It’s become common for several uncompromising senators to block debate on all other amendments to the NDAA if they cannot get guaranteed votes on their own proposals. Last year saw such a tangle between Republican Sens. Lindsey Graham, Chuck Grassley, Mike Lee and Rand Paul.

“We’ll try to have an open amendment process, but as you know, that’s contingent on people on both sides cooperating,” said the Senate majority whip, Sen. John Thune, R-S.D. “As soon as someone objects to not getting [a vote for] their amendment, you get into this lockdown.”

After forcing leaders on Thursday to run out the procedural clock on the NDAA, Paul suggested he would block other senators from getting amendment votes unless they agree to give him a vote on his amendments.

“I do believe we should demand an open debate with amendments,” he said.

Paul has filed six amendments to the bill, including two that have jammed up the process in the past. One would require a military pullout from Afghanistan and another would bar indefinite detention of enemy combatants.

Senate Armed Services Committee Chairman Jim Inhofe, R-Okla., will manage the bill’s progress, and he expressed concern Thursday that Paul could impede debate if he is dissatisfied.

“He could actually block this for a long period of time,” Inhofe, said of Paul. “If you remember last year, one bloc of amendments got through ― and there were countless amendments from Democrats and Republicans [that didn’t].”

To name a small fraction this year, lawmakers are hoping for votes on chemical contamination at military bases; a boost to F-35 aircraft procurement; expanded Iran sanctions; U.S. arms sales to Taiwan; increased aid to Israeli missile defense; restrictions on surveillance equipment exports to China; a buy-America requirement for the military’s eating utensils; restrictions on arms sales and U.S. nuclear cooperation with Saudi Arabia; reimposing sanctions on North Korea; and the politically sensitive topic of election security.

Some amendments reflected Democrats’ frustration with President Donald Trump’s deployment of troops to the southern border and use of emergency authorities to shift military funding to border barriers Congress did not authorize.

Per an amendment from the Senate’s No. 2 Democrat and a lead defense appropriator, Sen. Dick Durbin of Illinois, national defense funds would be barred from use for constructing barriers at the southern border. Civilian law enforcement agencies would have to repay the Defense Department for its support, per a proposed amendment from Sen. Richard Blumenthal, D-Conn. Military construction funding would be limited to $500 million under a declaration of war or national emergency, per a proposal from Sen. Brian Schatz, D-Hawaii.

On the flip side, the Air Force’s excess General Atomics MQ-1 Predator drones could be demilitarized and transferred to U.S. Customs and Border Patrol, per a proposed amendment by Thune. (Ever since the MQ-1 has been used on the border, it’s aided in few apprehensions of illegal border crossers, per CBP’s data.)

On the Middle East, lawmakers expressed their frustrations both with the president’s tough stance on Iran and his embrace of Saudi Arabia. Democrats have also expressed frustration at the president’s end run around Congress to aid Riyadh in spite of human rights concerns as well as fears he will not seek authorization to go to war with Iran, but rather unilaterally act.

Vermont Independent Sen. Bernie Sanders, a presidential candidate, proposed a ban on offensive combat operations against Iran without the express authorization of Congress and a separate ban on U.S. military support for the Saudi-led coalition fighting the Houthi rebels in Yemen. Per another amendment from Durbin, all war authorizations would sunset after 10 years.

On nuclear weapons, advocates of restraint are hoping for a vote on whether to bar the U.S. from a nuclear first strike. Separately, Sen. Elizabeth Warren, D-Mass., and a 2020 presidential contender, is proposing a ban on deployment on the W76-2 submarine-launched nuclear warhead. The language parallels a provision added to the House Armed Services Committee’s bill over GOP objections.

Military family advocates are hoping for a floor debate on plans to end the "widow's tax," a problem where thousands of dollars in payouts are withheld from surviving spouses because of conflicts in federal benefits law. The provision to fix the issue has 75 Senate co-sponsors, but also a price tag of nearly $6 billion that has kept the idea out of serious consideration in recent years.

After non-germane amendments were screened out, Inhofe hoped there would be 30 votes. Lawmakers will typically attempt to make bipartisan deals on packages of noncontroversial amendments.

As of Thursday, Inhofe and the panel’s ranking member, Sen. Jack Reed, D-R.I., had negotiated a package of 93 amendments, divided evenly between Republicans and Democrats, which could be approved as a bloc.

“We’ve had progress there and we’re trying to work on floor amendments, see what we can do there,” Reed said.

Any delays and obstruction will not be welcome by leaders. Thune anticipated the July 4 recess would provide a backstop and greater sense of urgency to pass the bill by week’s end.

“Hopefully that will generate a willingness on the part of individual senators to come forward and work together to get as many amendments cleared as possible and hopefully get to an ultimate vote by the end of the week,” Thune said.

Leo Shane III at Military Times contributed to this report.