I could use some help on understanding the Cleric's spiritual weapon spell as it caused a serious point of contention in my group's last session. The issue was whether or not the target of the spiritual weapon spell had to be within the cleric's melee range (5ft) in order to make the successful melee spell attack required. 

The party was surrounded by a horde of zombies and the cleric was attempting to caste it on a mage ~ 30ft away, outside the ring of zombies. My prior experience with melee spell attacks (Booming Blade, Green Flame Blade) made me think that the cleric needed to physically connect with the target, but the wording of the spell in regards to summoning range and subsequently moving the spiritual weapon complicated things.

So, could the cleric have summoned the weapon right next to the mage (30 ft.) and smacked him with it? I would assume if he was casting through the horde he would make the roll at a disadvantage because of the hindered line of site, right? Any help would be appreciated, the party ended up all going down to 0 hp, and this attack probably would have turned the tide.

Thanks

Yes, he absolutely can.  You're confusing "melee spell attack" with Touch spells.

You create a floating, spectral weapon within range (60 feet.) ... When you cast the spell, you can make a melee spell attack against a creature within 5 feet of the weapon.

As a bonus action on your turn, you can move the weapon up to 20 feet and repeat the attack against a creature within 5 feet of it.

"It," as in the the weapon; not the caster.  The caster would not have disadvantage on this attack, even with enemies next to him.  The only time you have disadvantage on an attack because enemies are next to you is when you are making a ranged attack, which this is not.

You don't even need to have line of sight at all; this spell does not specify that you need to see where it appears, just that you summon it within range (60 feet).  This spell is usable while completely blind.

 Not exactly confusing "melee spell attack" with "Touch," necessarily, but yes Jays is right. 

Spells don't unfortunately always use the same tools to tell you what their range is. Booming Blade, Shocking Grasp, and Vampiric Touch, for example, all pretty much let you make a melee attack (a melee weapon attack for Booming Blade, a melee spell attack for Shocking Grasp and Vampiric Touch) against an enemy within 5 feet. BB tells you that its within 5 feet, because it gives you "5 feet" as a range and tells you to "make a melee attack with a weapon against one creature within the spell’s range". SG tells you that its within 5 feet, because it tells you that it's range is "Touch," and touch is defined as what you can... touch in Chapter 10, which for most medium creatures is 5 feet. VT instead tells you that its range is "Self," but then empowers you to make melee spell attacks at enemies within your "reach", which again for most medium creatures is 5 feet. Three spells, three completely different configurations of "range," and yet effectively they all mean "5 feet."

Spiritual Weapon tells you that its range is 60 feet, and you should take that seriously. The first sentence of the spell ("You create a spectral weapon within range...") tells you what that 60 feet is about. It then says you can "make a melee spell attack against a creature within 5 feet of the weapon" you've created, which is pretty clear. Even if this spell doesn't necessarily resemble any other spell you find to compare it to, if you just take the time to read through the wording of the spell, it shouldn't steer you wrong. 

Awesome, thank you for clarifying that in depth. Much appreciated.